Low defect group III nitride films useful for electronic and optoelectronic devices and methods for making the same

ABSTRACT

In a method for making a low-defect single-crystal GaN film, an epitaxial nitride layer is deposited on a substrate. A first GaN layer is grown on the epitaxial nitride layer by HVPE under a growth condition that promotes the formation of pits, wherein after growing the first GaN layer the GaN film surface morphology is rough and pitted. A second GaN layer is grown on the first GaN layer to form a GaN film on the substrate. The second GaN layer is grown by HVPE under a growth condition that promotes filling of the pits, and after growing the second GaN layer the GaN film surface morphology is essentially pit-free. A GaN film having a characteristic dimension of about 2 inches or greater, and a thickness normal ranging from approximately 10 to approximately 250 microns, includes a pit-free surface, the threading dislocation density being less than 1×10 8  cm −2 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/749,728, filed Dec. 12, 2005, titled “BulkGallium Nitride Crystals and Method of Making the Same;” U.S.Provisional Patent Application Ser. No. 60/750,982, filed Dec. 16, 2005,titled “Method of Producing Freestanding Gallium Nitride bySelf-Separation;” U.S. Provisional Patent Application Ser. No.60/810,537, filed Jun. 2, 2006, titled “Low Defect GaN Films Useful forElectronic and Optoelectronic Devices and Method of Making the Same;”U.S. Provisional Patent Application Ser. No. 60/843,036, filed Sep. 8,2006, titled “Methods for Making Inclusion-Free Uniform Semi-InsulatingGallium Nitride Substrate;” and U.S. Provisional Patent Application Ser.No. 60/847,855, filed Sep. 28, 2006, titled “Method of Producing SingleCrystal Gallium Nitride Substrates by HVPE Method Incorporating aPolycrystalline Layer for Yield Enhancement,” the contents of which areincorporated by reference herein in their entireties.

BACKGROUND

1. Field of the Invention

This invention relates to low-defect density, device-quality galliumnitride (Al, Ga, In)N films useful for producing electronic andoptoelectronic devices, such as high electron mobility transistors(HEMTs), heterojunction bipolar transistors (HBTs), light emittingdiodes (blue, UV and white LEDs), and laser diodes (LDs). The inventionalso relates to methods for producing such GaN films.

2. Description of the Related Art

Group III-V nitride compounds such as aluminum nitride (AlN), galliumnitride (GaN), indium nitride (InN), and alloys such as AlGaN, InGaN,and AlGaInN, are direct bandgap semiconductors with bandgap energyranging from about 0.6 eV for InN to about 6.2 eV for AlN. Thesematerials may be employed to produce light emitting devices such as LEDsand LDs in short wavelength in the green, blue and ultraviolet (UV)spectra. Blue and violet laser diodes may be used for reading data fromand writing data to high-density optical data storage discs, such asthose used by Blu-Ray and HD-DVD systems. By using proper colorconversion with phosphors, blue and UV light emitting diodes may be madeto emit white light, which may be used for energy efficient solid-statelight sources. Alloys with higher bandgaps can be used for UVphotodetectors that are insensitive to solar radiation. The materialproperties of the III-V nitride compounds are also suitable forfabrication of electronic devices that can be operated at highertemperature, or higher power, and higher frequency than conventionaldevices based on silicon (Si) or gallium arsenide (GaAs).

Most of the III-V nitride devices are grown on foreign substrates suchas sapphire (Al₂O₃) and silicon carbide (SiC) because of the lack ofavailable low-cost, high-quality, large-area native substrates such asGaN substrates. Blue LEDs are mostly grown on insulating sapphiresubstrates or semi-conducting silicon carbide substrates using ametal-organic chemical vapor deposition (MOCVD) process.

The MOCVD process is a slow growth rate process with a growth rate of afew microns per hour. In a typical GaN-based device growth process, alow-temperature buffer layer of GaN or Al_(x)Ga_(1-x)N (x=0-1) is firstgrown on a foreign substrate (e.g., sapphire or silicon carbide),followed by the growth of a few microns of GaN. The active device layer,such as quantum well structures for LEDs, is subsequently grown. Forexample, U.S. Pat. No. 5,563,422 to S. Nakamura et al. describes aGaN-based device grown by an MOCVD process. A thin GaN nucleation layerof about 10 nanometers is first deposited on a sapphire substrate at alow temperature of 500-600° C. The GaN nucleation layer is annealed athigh temperature to recrystallize the GaN, and epitaxial GaN film isgrown at higher temperature (approximately 1000-1200° C.).

Because of the lattice mismatch between gallium nitride and thenon-native substrate, there is a large number of crystal defects in theGaN film and active device layer. The defect density in the GaNnucleation layer is thought to be on the order of 10¹¹ cm⁻² or greater,and in the subsequently grown GaN layer and active device layer, thetypical density of crystal defects, in particular, the threadingdislocation density, is on the order of 10⁹-10¹⁰ cm⁻² or greater intypical GaN-based LEDs. Despite the high defect density of LEDs grown onthese substrates, commercial low-power blue/white LEDs have longlifetimes suitable for some applications.

Group III-V nitride-based laser diodes, however, show a remarkabledependence of lifetime on the crystal defect density. The lifetime ofthese LDs dramatically decreases with the increase of the dislocationdensity (see, for example, “Structural defects related issues ofGaN-based laser diodes,” S. Tomiya et al., MRS Symposium Proceedings,Vol. 831, p. 3-13, 2005). Low-defect density single-crystal galliumnitride is needed for the long lifetime (>10,000 hours) nitride laserdiodes. For LEDs based on an AlGaN active layer operating at the deeperUV range, it is also found that dislocation density has a detrimentaleffect on the performance and lifetime of the devices. For LEDsoperating at higher power levels, it is also desirable to have a lowerdefect density GaN layer.

There are several growth methods that may possibly be performed toreduce the defect density of the gallium nitride film. One commonapproach in MOCVD growth of gallium nitride is epitaxial lateralovergrowth (ELOG) and its variations. In an ELOG GaN growth process, aGaN film is first grown by a MOCVD method with the 2-step process(low-temperature buffer and high-temperature growth). A dielectric layersuch as silicon oxide or silicon nitride is deposited on the first GaNfilm. The dielectric layer is patterned with a photolithographic methodand etched so that portions of GaN surface are exposed and portions ofthe GaN film are still covered with the dielectric mask layer. Thepatterned GaN film is reloaded into the MOCVD reactor and growth isre-commenced. The growth condition is chosen such that the second GaNlayer can only be grown on the exposed GaN surface, but not directly onthe masked area. When the thickness of the second GaN layer is thickerthan the dielectric layer, GaN can grow not only along the original cdirection, but also along the sidewalls of the GaN growing out of theexposed area and gradually covering the dielectric mask. At the end ofthe growth, the dielectric mask will be completely covered by the GaNfilm and the GaN film overall is quite smooth. However, the distributionof the threading dislocation density is not uniform. Since thedislocation density of the first GaN layer is quite high, the defectdensity is also high in the area of the second GaN layer grown directlyon the exposed first GaN layer. In comparison, the defect density ismuch reduced in the area above the dielectric mask area where the secondGaN layer was grown laterally in the direction parallel to the surface.The defect density is still high in the area where the second GaN layerwas grown directly on the first GaN layer and in the area where thelateral grown GaN coalesced. Multiple ELOG processes can be used tofurther reduce the defect density by patterning a second dielectric maskcovering the high defect density areas of the first ELOG GaN film, andgrowing GaN film in the ELOG condition that yields a coalesced secondELOG film.

The manufacturing cost of the prior-art low defect density GaN filmbased on MOCVD is high due to multiple growth and photolithographicsteps. The high cost of the film also increases the overallmanufacturing cost of end products such as UV LEDs.

Therefore, there is still a compelling need in the art for a low-costmethod of producing high-quality, low defect density GaN films that aresuitable for electronic and optoelectronic devices to be built on.

SUMMARY

The present invention generally relates to high-quality gallium nitride(Al, Ga, In)N films (articles, substrates, layers, etc.) and methods formaking the same.

According to one implementation, a method for making a low-defectsingle-crystal gallium nitride (GaN) film is provided. An epitaxialaluminum nitride (AlN) layer is deposited on a substrate. A firstepitaxial GaN layer is grown on the AlN layer by HVPE under a growthcondition that promotes the formation of pits, wherein after growing thefirst GaN layer the GaN film surface morphology is rough and pitted. Asecond epitaxial GaN layer is grown on the first GaN layer to form a GaNfilm on the substrate. The second GaN layer is grown by HVPE under agrowth condition that promotes filling of the pits, and after growingthe second GaN layer the GaN film surface morphology is essentiallypit-free.

According to another implementation, the GaN growth condition forgrowing the first GaN layer is selected from the group consisting of ahigher growth rate than during growth of the second GaN layer, a lowergrowth temperature than during growth of the second GaN layer, a higherammonia flow than during growth of the second GaN layer, and two or moreof the foregoing.

According to another implementation, a low-defect single-crystal GaNfilm produced according to any of the above methods is provided.

According to another implementation, a low-defect single-crystal GaNfilm is provided. The GaN film has a characteristic dimension of about 2inches or greater, and a thickness normal to the characteristicdimension ranging from approximately 10 to approximately 250 microns.The GaN film includes a pit-free surface. The threading dislocationdensity on the GaN film surface being less than 1×10⁸ cm⁻².

According to another implementation, low-defect single-crystal galliumnitride (GaN) on substrate structure is provided. The structure includesa substrate, an epitaxial aluminum nitride (AlN) layer on the substrate,and a GaN film on the substrate. The GaN film includes a first epitaxialGaN growth layer and a second epitaxial GaN growth layer. The firstepitaxial GaN layer is grown on the AlN layer under a growth conditionthat promotes the formation of pits, and after growing the first GaNlayer the GaN film surface morphology is rough and pitted. The secondepitaxial GaN is grown on the first GaN layer by HVPE under a growthcondition that promotes filling of the pits formed, and after growingthe second GaN layer the GaN film surface morphology is essentiallypit-free.

According to any of the above implementations, the threading dislocationdensity on the GaN film surface is minimal. In one example, thethreading dislocation density on the surface of the GaN film may be lessthan 1×10⁸ cm⁻², in another example less than 5×10⁷ cm⁻², in anotherexample less than 1×10⁷ cm⁻², and in another example less than 5×10⁶cm⁻².

According to any of the above implementations, the amount of bowing theGaN film on an underlying substrate is minimal. In one example, the bowof the GaN film may be less than about 200 microns. In another example,the bow of the GaN film may be less than about 100 microns. In anotherexample, the bow of the GaN film may be less than about 50 microns. Inanother example, the bow of the GaN film may be less than about 25microns.

According to any of the above implementations, the surface of the GaNfilm may have a root-mean square (RMS) surface roughness of about 0.5 nmor less.

Other aspects, features and embodiments of the invention will be morefully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a vertical HVPE reactor.

FIG. 2 is an optical micrograph at 50× magnification of the surface of aGaN film grown on an AlN-coated sapphire substrate under a typical HVPEGaN growth condition. The GaN film thickness was approximately 1 micron.

FIG. 3 is an optical micrograph at 50× magnification of a GaN film grownon an AlN-coated sapphire substrate under the same condition as the filmshown in FIG. 2, but to a thickness of approximately 5 microns.Microcracking of the film is visible.

FIG. 4 is an optical micrograph at 50× magnification of a pitted GaNfilm, approximately 110 microns thick, grown on AlN-coated sapphiresubstrate under a moderate NH₃ partial pressure growth condition.

FIG. 5 is a schematic illustration of an example of a growth process ofthe present invention.

FIG. 6 is an optical micrograph at 200× magnification of the surface ofa GaN layer at the end of a growth step in which the surface is pitted,according to one implementation of the present invention.

FIG. 7 is a cross-sectional view of a bowed wafer or layer of material.

FIG. 8 is an optical micrograph at 50× magnification of the surface ofan as-grown 60-micron GaN film on sapphire substrate, according to oneimplementation of the present invention.

FIG. 9 is a 10×10-micron AFM scan of a 60-micron thick GaN film onsapphire, grown according to one implementation of the presentinvention.

DETAILED DESCRIPTION

Throughout the disclosure, unless otherwise specified, certain terms areused as follows. “Single crystalline film” or “single crystal” means acrystalline structure that can be characterized with x-ray rocking curvemeasurement. The narrower the peak of the rocking curve, the better thecrystal quality. “Single crystal” does not necessarily mean that thewhole crystal is a single grain; it may contain many crystalline grainswith orientation more or less aligned. “Polycrystalline film” or“polycrystal” means that a crystal has many grains whose crystalorientations are randomly distributed. An X-ray rocking curvemeasurement of a polycrystalline film does not exhibit a peak.“Microcracks” are a cluster of localized cracks with high density ofcracks. The distance between the parallel cracks in the microcrackcluster is typically less than 100 microns. “Growth cracks” are thecracks formed during crystal growth. “Cool down cracks” or “thermalcracks” are the cracks formed after the crystal growth and during thecooling of the crystal from the growth temperature to ambient or roomtemperature. “Pits” are typically inverse pyramidal pits on the crystalsurface. “Pit-free surface” is a surface essentially having no pits onits surface. “Pitted surface morphology” means a surface having asubstantial amount of pits on its surface. “Faceted surface morphology”means that a single crystal film surface is completely covered with pitsso that the sides of the pits become the surface itself and the surfaceappears faceted. “Smooth surface morphology” means that a surface isspecular and has no visual defects (such as pits). “Nucleation layer” or“template layer” in some implementations may be the layer first grown ona substrate. “V:III ratio” in some implementations is the ratio of theammonia flow to the HCl flow used during a hydride vapor phase epitaxyGaN growth process. “Ammonia partial pressure” is calculated accordingto the ammonia flow, the total gas flow into a reactor, and the reactorpressure. “Growth surface” or “growing surface” or “growth front” is thesurface of the GaN crystal during the instance of the growth.

For purposes of the present disclosure, it will be understood that whena layer (or film, region, substrate, component, device, or the like) isreferred to as being “on” or “over” another layer, that layer may bedirectly or actually on (or over) the other layer or, alternatively,intervening layers (e.g., buffer layers, transition layers, interlayers,sacrificial layers, etch-stop layers, masks, electrodes, interconnects,contacts, or the like) may also be present. A layer that is “directlyon” another layer means that no intervening layer is present, unlessotherwise indicated. It will also be understood that when a layer isreferred to as being “on” (or “over”) another layer, that layer maycover the entire surface of the other layer or only a portion of theother layer. It will be further understood that terms such as “formedon” or “disposed on” are not intended to introduce any limitationsrelating to particular methods of material transport, deposition,fabrication, surface treatment, or physical, chemical, or ionic bondingor interaction.

Unless otherwise indicated, terms such as “gallium nitride” and “GaN”are intended to describe binary, ternary, and quaternary Group IIInitride-based compounds such as, for example, gallium nitride, indiumnitride, aluminum nitride, aluminum gallium nitride, indium galliumnitride, indium aluminum nitride, and aluminum indium gallium nitride,and alloys, mixtures, or combinations of the foregoing, with or withoutadded dopants, impurities or trace components, as well as all possiblecrystalline structures and morphologies, and any derivatives or modifiedcompositions of the foregoing. Unless otherwise indicated, no limitationis placed on the stoichiometries of these compounds.

Single-crystal GaN films can be grown on sapphire substrates withvarious vapor phase growth techniques, such as molecular beam epitaxy(MBE), metal-organic vapor phase epitaxy (MOVPE), and hydride vaporphase epitaxy (HVPE). In the MBE and MOVPE growth of GaN films onsapphire, a low-temperature buffer layer is typically needed to growhigh-quality GaN film. It is not clear whether a buffer layer is neededfor HVPE GaN growth on sapphire. Lee in U.S. Pat. No. 6,528,394discloses a specific method of pre-treatment for growing GaN on sapphireusing HVPE. The pre-treatment involves etching sapphire with a gasmixture of hydrochloric acid (HCl) and ammonia (NH₃), as well asnitridation of the sapphire substrate. Molnar in U.S. Pat. No. 6,086,673discloses the use of a zinc oxide (ZnO) pretreatment layer that wasfurther reacted in the gaseous environment of HCl and/or NH₃. After thistreatment of sapphire substrate, single-crystal GaN film is then grownby HVPE. On the other hand, Vaudo et al in U.S. Pat. No. 6,440,823discloses the growth of a low defect density GaN layer on sapphire bythe HVPE method, without using any buffer layers or nucleation layers.

Since teachings in the prior art regarding sapphire substrate treatmentor initiation prior to HVPE GaN growth are in conflict, wesystematically investigated the growth of gallium nitride film onsapphire using an HVPE process. Vertical HVPE reactors were used for theinvestigation. FIG. 1 schematically illustrates an example of a verticalHVPE reactor 100. The HVPE reactor 100 includes a quartz reactor tube104 that is heated by a multi-zone furnace 108. The reactor tube 104 isconnected to gas inlets 112, 116, and 120 for introducing reactants,carrier gases, and diluting gases. The reactor tube 104 is alsoconnected to a pump and exhaust system 124. In some implementations,inside the reactor 100, gaseous hydrochloric acid (HCl) is flowedthrough a vessel 128 containing gallium metal 132, which is at atemperature of, for example, about 850° C. The hydrochloric acid reactswith the gallium metal 132, forming gaseous GaCl, which is transportedby a carrier gas, such as nitrogen, to the deposition zone in thereactor tube 104. Ammonia (NH₃) and an inert diluent gas, such asnitrogen, are also flowed to the deposition zone where GaN crystals aredeposited. The reactor 100 is designed such that the mixing of GaCl andNH₃ does not occur near the gas outlets, ensuring no deposition of GaNon the outlets of GaCl and NH₃ and enabling long-term stability of gasflow patterns. Epi-ready c-plane sapphire substrates or other suitablesubstrates 136 may be used. The substrate 136 is placed on a rotatingplatter 140 and heated to a temperature of, for example, 900-1100° C.

A typical deposition run process is as follows: (1) a substrate 136 isplaced on the platter 140, (2) the reactor 100 is sealed, (3) thereactor 100 is evacuated and purged with high-purity nitrogen to removeany impurities from the system, (4) the platter 140 with the substrate136 is raised to the deposition zone, (5) the platter temperature iscontrolled at the desired deposition temperature, (6) ammonia is flowedinto the reactor 100, (7) HCl is flowed to the reactor 100 to start theGaN deposition, (8) deposition proceeds according to a predeterminedrecipe for a predetermined time, (9) the HCl and NH₃ gas flows arestopped, (10) the platter 140 is lowered and the grown crystal isgradually cooled down, and (11) the grown crystal is removed forcharacterization and further processing.

After systematically investigating the HVPE growth of GaN on sapphiresubstrates, we uncovered several issues that were not disclosed in theprior art, namely, irreproducible nucleation of single crystal GaN filmson untreated sapphire substrates, and microcracking of singlecrystalline GaN films.

First, we grew various HVPE GaN films directly on sapphire substrateswithout any buffer layer or pretreatment under the conditions taught bythe prior art, i.e., a growth temperature of about 950-1050° C., V:IIIratio (i.e., NH₃/HCl) of about 10-50, and a growth rate of about 100microns per hour. The bare sapphire substrate was heated up to thegrowth temperature, ammonia flow was turned on first to fill the reactorto a pre-determined partial pressure and HCl flow was turned on toinitiate the growth. The GaN film grown directly on the bare sapphiresubstrate was not smooth. After analyzing the grown GaN films with anx-ray rocking curve and optical microscope, we determined that the GaNfilms grown directly on bare sapphire substrates were notsingle-crystalline films. In fact, they were polycrystalline GaN. Wewish not to be bound by any particular theory regarding the variousresults of HVPE GaN crystal growth on sapphire, but the discrepancy inthe various prior-art work and our own work may be related to particularreactor configurations or surface treatments. The prior art did notteach a reproducible method to grow single crystal GaN films on sapphiresubstrate by HVPE.

There is a large lattice mismatch between sapphire and gallium nitride.Furthermore, c-plane GaN is a polar crystal, i.e., one face isterminated with gallium and the opposite face of the crystal isterminated with nitrogen. On the other hand, sapphire is not a polarcrystal; the c-plane of sapphire is terminated with oxygen on bothfaces. In other GaN thin-film deposition techniques such as molecularbeam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD), athin buffer layer is required for the high-quality single-crystallineGaN growth. The buffer layer may be an AlN layer (S. Yoshida et al.,Appl. Phys. Lett., 42, 427 (1983); H. Amano et al., Appl. Phys. Lett.,48, 353 (1986)) or a GaN layer grown at low temperature (S. Nakamura,Jpn. J. Appl. Phys., 30. L1705 (1991)). Lee in U.S. Pat. No. 6,528,394postulated the formation of a thin AlN layer on the sapphire surface bythe pre-treatment step prior to HVPE GaN growth.

U.S. Pat. No. 6,784,085, the entire contents of which are incorporatedinto the present disclosure, discloses a high-temperature reactivesputtering method for growing high-quality AlN film on sapphiresubstrates. Using this method, we coated sapphire substrates with AlNfor use as substrates for HVPE GaN growth.

High-quality GaN thin films were successfully and reproducibly grown onthe AlN-coated sapphire substrate. We first grew a thin layer of AlNfilm on a sapphire substrate by sputtering using the method disclosed inU.S. Pat. No. 6,784,085. The typical thickness of the AlN layer wasapproximately 0.5-2 microns. X-ray rocking curve measurement indicatedthe AlN film was an epitaxial and single-crystalline film with (0002)rocking curve full width at half maximum (FWHM) of 50 arcsec. TheAlN-coated sapphire substrate was loaded into the HVPE reactor and a GaNfilm was grown using the aforementioned procedure. The growth rate wasabout 60 microns per hour, the GaCl partial pressure was about 2.97Torr, the NH₃ partial pressure was about 44.6 Torr, the V:III ratio wasabout 15, and the growth temperature was about 950° C. as measured witha thermocouple under the platter. The growth time was 1 minute. The GaNfilm grown was transparent with a smooth, specular surface. FIG. 2 showsan optical micrograph of the surface of the GaN film. FIG. 2 shows atypical smooth surface morphology for an HVPE GaN film with some hillockfeatures. X-ray rocking curve measurements confirm thesingle-crystalline nature of the GaN film, with a FWHM value of 297arcsec.

After developing a nucleation process for GaN single crystalline filmson AlN sputter-coated sapphire substrates, we investigated the growth ofthicker GaN films. We discovered a problem, namely, microcracking in theGaN films. The HVPE growth conditions were chosen to produce a smoothGaN surface. FIG. 3 shows an optical micrograph of thin GaN film,approximately 5 microns thick on an AlN-coated sapphire substrate, grownunder the same conditions as the film shown in FIG. 2. The surfaceexhibits a typical smooth HVPE GaN morphology with hillock features.However, microcracks in the GaN film are apparent. The sapphiresubstrate remains intact without any cracking in this case.

Because of the difference between the coefficients of thermal expansionof the sapphire substrate and the GaN film, thermal stress builds upwhen the film cools down from the typical growth temperature of about1000° C. to ambient room temperature. As discussed in open literature(for example, E. V. Etzkom and D. R. Clarke, “Cracking of GaN Films,” J.Appl. Phys., 89 (2001) 1025), sapphire substrate shrinks faster than GaNfilm during cool down, causing a compressive stress in the GaN film dueto this thermal expansion mismatch. The compressive thermal stress inthe GaN film should not cause microcracking in the GaN film during cooldown. Therefore, the microcracks must be already formed during the GaNgrowth and prior to cool down.

The microcracking of the GaN film during the growth suggests a tensilestress in the GaN film during the growth. We wish not be bound by anyparticular theory regarding the origin of microcracking during GaNgrowth. However, the tensile stress may be related to the AlN layeremployed in the study, or may be related to the HVPE growth conditionused, or may be universal to the vapor phase GaN growth in general.While cracking is noted in some instances, most prior-art literature inHVPE GaN growth does not disclose the formation of microcracks in GaNfilm during growth. The prior art also does not teach how to eliminatethe microcracks during the HVPE GaN growth.

In order to eliminate the microcracks formed during the HVPE GaN growth,we systematically investigated GaN growth on the AlN-coated sapphireunder various growth conditions by varying growth parameters, such asGaCl flow or partial pressure (which may be determined by the flow ofhydrochloric acid (HCl)), NH₃ flow or partial pressure, growthtemperature, and associated variables such as growth rate and V:IIIratio (e.g., NH₃/HCl ratio). In this example, the V:III ratio is theratio of the NH₃/HCl flow. The growth rate is typically proportional toGaCl partial pressure, which is directly related to the HCl flow. Wefound that the surface morphology of the GaN film varies substantiallywith the growth temperature, growth rate and ammonia partial pressure(or V:III ratio). At a constant growth temperature and GaCl partialpressure, increasing NH₃ partial pressure dramatically alters behaviorof microcracking and surface morphology. For a constant growth time(similar film thickness, about 100 microns), the HVPE GaN surfacemorphology gradually changes from a smooth, hillocked morphology withmicrocracks at low NH₃ partial pressure, to a surface covered with pitsat moderately high NH₃ partial pressure, and eventually topolycrystalline material at high NH₃ partial pressure. When the GaN filmis covered with pits, the microcracks formed during the growth are alsoeliminated.

FIG. 4 is a micrograph of a GaN surface grown under moderately high NH₃partial pressure (moderate V:III ratio). This particular GaN film wasgrown on an AlN-coated sapphire substrate. The growth rate was about 320microns per hour, the GaCl partial pressure was around 1.8 Torr, the NH₃partial pressure was around 112.8 Torr, the V:III ratio was around 58,and the growth temperature was about 990° C. The growth time was 20minutes. Although the GaN film surface is covered with pits, the film isstill epitaxial single-crystalline film, as confirmed by x-ray rockingcurve measurement, with FWHM of 400 arcsec. The larger FWHM value of thefilm is due in part to curvature of the sample, which is known tobroaden the X-ray diffraction peak.

Similar surface morphology trends are observed with growth temperatureat otherwise constant conditions, or with growth rate at otherwiseconstant conditions. Under constant GaCl and NH₃ partial pressures(constant growth rate and V:III ratio), reducing the growth temperaturealters the growth morphology from a smooth, hillocked structure to apitted surface morphology and eventually to a polycrystallinemorphology. Similarly, at a constant growth temperature and V:III ratio,increasing the growth rate (by increasing both GaCl and NH₃ partialpressure) alters the surface morphology from a smooth pit-free surfaceto a pitted surface morphology and eventually to a polycrystallinemorphology.

The pitted surface morphology eliminates the microcracks in the GaN filmduring the HVPE GaN growth. However, the surface is not desirable as afoundation of further growth of GaN-based device structures. The presentinvention discloses methods for growing high-quality, low defectdensity, pit-free and crack-free GaN films by hydride vapor phaseepitaxy. The GaN films are suitable for the further growth of electronicand optoelectronic devices based on group III nitride alloys.

The GaN growth method of the present invention may include severalgrowth steps, including depositing an epitaxial nitride template layeron a suitable substrate, growing a thin GaN layer on the nitride-coatedsubstrate under a condition that yields a surface covered with pits, andgrowing a GaN layer on or from the pitted GaN layer under a conditionthat fills the pits and yields a pit-free surface.

According to this implementation, the first step of the growth processis to deposit a thin epitaxial nitride (e.g., AlN) layer on a suitablesubstrate such as, for example, sapphire. The purpose of this epitaxialnitride layer is to provide a template for epitaxial growth of GaN.Without the epitaxial nitride template, the HVPE GaN film grown on asubstrate such as sapphire under typical conditions is polycrystalline.The epitaxial nitride layer in one implementation is prepared byhigh-temperature reactive sputtering in a sputtering chamber. Analuminum target and an AC plasma of an inert gas or gas mixture (e.g.,an Ar/N₂ gas mixture) may be utilized to deposit the epitaxial nitridelayer on a heated substrate. The epitaxial nitride layer mayalternatively be formed by molecular beam epitaxy (MBE), metal-organicvapor phase epitaxy (MOVPE or MOCVD), hydride vapor phase epitaxy, orhigh-temperature annealing in ammonia. In one example, the thickness ofthe epitaxial nitride layer is in the range (ranges) from about 0.05 toabout 2 microns. In another example, the thickness of the epitaxialnitride layer ranges from about 0.2 to about 2 microns. Other types oftemplate layers may alternatively be used, for example, GaN or AlGaNlayers, grown by MOVPE, MBE or HVPE.

The second step of the growth process is to grow a GaN layer by hydridevapor phase epitaxy in a growth condition that yields pitted surfacemorphology. The nitride-coated substrate is loaded into a HVPE reactor,and the reactor may be purged with high purity nitrogen to removeimpurities. A layer of gallium nitride is then grown on the epitaxialnitride layer. The growth condition for this GaN layer is typicallyhigher growth rate, and/or higher ammonia flow (or V:III ratio), and/orlower growth temperature than the “optimal” thin-film growth condition.The “optimal” thin film growth condition is one that would producesmooth, substantially pit-free, crack-free thin films (e.g., with athickness equal to or less than 3 microns), but would producemicrocracked thick films (e.g., with a thickness equal to or greaterthan 20 microns). As one specific example of an optimized growthcondition, a 1-micron thick GaN film that is transparent and has asmooth specular surface has been grown on an AlN-coated sapphiresubstrate by the inventors. The growth rate was about 60 microns perhour, the GaCl partial pressure was about 3 Torr, the NH₃ partialpressure was about 45 Torr, the V:III ratio was about 15, the growthtemperature was about 950° C., and the growth time was one minute. Whengrowing a thin film (≦3 μm), this “optimal” thin-film growth conditiontypically produces a crack-free film, whereas when growing a thick film(≧20 μm), the “optimal” growth condition typically produces amicrocracked film.

The GaN film grown under the growth condition of this second step isvery rough and covered with pits. There are two purposes for this pittedlayer: first is to prevent microcracking of GaN during the growth; andsecond is to promote annihilation of dislocations. In one example, thethickness of this pitted layer ranges from approximately 2 toapproximately 50 microns. In another example, the thickness of thispitted layer ranges from approximately 5 to approximately 50 microns. Ifthe GaN film is grown under the pitted growth condition with higherthickness, the GaN film quality is gradually changed from an epitaxialsingle-crystalline film to a polycrystalline film.

In one implementation, the growth temperature during growth of the first(pitted) epitaxial GaN layer ranges from about 900° C. to about 1000°C., the V:III ratio ranges from about 10 to about 100, and the growthrate ranges from about 50 μm/hr to about 500 μm/hr.

The third step of the growth process is to grow an additional GaN layerunder conditions that cause the pits to be filled and yield a pit-freeand crack-free surface. The growth condition for this layer is typicallylower growth rate, and/or lower ammonia partial pressure, and/or highergrowth temperature than the growth condition utilized for the pittedlayer. The thickness of this layer is in one example greater than about3 microns, in another example greater than about 5 microns, and inanother example greater than about 10 microns. In another example, thethickness of second epitaxial GaN layer ranges from about 3 to about 200microns. In another example, the thickness of second epitaxial GaN layerranges from about 8 to about 200 microns. The optimal thickness of thepit-free layer depends on the thickness of the pitted layer. A thickerlayer grown under pitted growth conditions correspondingly requires athicker layer grown under pit-free conditions to completely fill thepits. The ratio of the thickness of the layer grown under pittedconditions to the thickness of the layer grown under pit-free conditionis in one example between about 2:1 and about 1:5.

In one implementation, the growth temperature during growth of thesecond epitaxial GaN layer ranges from about 920° C. to about 1100° C.,the V:III ratio ranges from about 8 to about 80, and the growth rateranges from about 5 μm/hr to about 500 μm/hr.

FIG. 5 is a schematic illustration of an exemplary growth process 500 ofthe present invention. First, a substrate 504 is provided. An epitaxialnitride (e.g., AlN) layer 508 is then deposited on the substrate 504.The deposition of the epitaxial nitride layer 508 may be done in thesame reactor for the subsequent GaN growth or in a different depositionchamber. GaN material is subsequently deposited on the nitride-coatedsubstrate 504/508 by hydride vapor phase epitaxy in two steps withdifferent growth conditions. A first GaN layer 512 is grown under acondition that results in a pitted surface morphology, and suchconditions are characterized by relatively higher growth rate, and/orhigh ammonia flow, and/or lower growth temperature than utilized duringthe second GaN growth step. A second GaN layer 516 is then grown under acondition that fills the pits on the surface 514 of the first GaN layer512 and yields pit-free smooth GaN layer, and such growth conditions arecharacterized by relatively lower growth rate, and/or lower ammoniaflow, and/or higher growth temperature than employed in the first,pitted-growth step. The combination of the two GaN growth steps botheliminates the GaN microcracking during the growth and provides asmooth, low-defect GaN surface 518 that is suitable for the furthergrowth of devices based on Group III nitrides. The growth process 500yields a GaN film generally depicted at 524 in FIG. 5.

The substrate 504 may be any substrate that has a surface having a3-fold symmetry or close to having a 3-fold symmetry. Some examples ofthe present disclosure utilize c-plane sapphire as the substrate 504.Other substrates 504 such as silicon, silicon carbide, diamond, lithiumgallate, lithium aluminate, zinc oxide, spinel, magnesium oxide, andgallium arsenide may be utilized for the growth of low-defect,crack-free GaN films. In one example, the substrate 504 has acharacteristic dimension (e.g., diameter) of about 2 inches or greater.In other examples, the diameter of the substrate 504 is about 2″ orgreater, about 3″ or greater, about 4″ or greater, or any other suitablesize.

The substrate surface 506 may be exactly c-plane or vicinal surfaces ofthe c-plane. Vicinal surfaces may promote step-flow during the HVPE GaNgrowth and may yield smoother surface morphology. The offcut angle ofthe vicinal surface with respect to the c-plane in one example rangesfrom about 0° to about 10°, in another example from about 0.10 to about100, and in another example from about 0.50 to about 5°. The directionof offcut may be along the <1-100> direction or along the <11-20>direction, or along a direction between <1-100> and <11-20>.

In some implementations, the deposition of the epitaxial nitride layer508 may be needed to grow single-crystalline GaN films on substrates 504such as sapphire substrates using the HVPE process. In oneimplementation, the epitaxial nitride layer 508 is deposited by reactivesputtering on a heated substrate 504 in a sputter deposition chamber.The nitride-coated substrate 504/508 is subsequently removed from thesputter chamber and loaded into the HVPE reactor for GaN growth. Asalternatives to depositing AlN by HVPE, other nitride layers, such asAlN grown by MOCVD, GaN grown by MOCVD, AlGaN grown by MOCVD, and thelike may also be used. A reactive sputtering-deposited AlN layer has theadvantage of lower cost than MOCVD or MBE deposited nitride layers. AlNlayers may also be grown in the HVPE reactor by incorporating an Alsource so that hydrochloric acid reacts with Al to form aluminumchloride that reacts with ammonia in the deposition zone to form AlN onthe substrate surface 506.

The growth of GaN film 524 according to this implementation includes atleast two growth steps with different growth conditions. The growthtemperature is typically between 900° C. and 1100° C., the growth rateis typically between 5 and 500 microns per hour, and V:III ratio istypically between 5 and 100. The two-step GaN growth is characterized bythe growth conditions of the first step having lower growth temperature,and/or higher ammonia flow, and/or higher growth rate than the secondstep. In one example, the growth temperature is about 15° C. hotter inthe second step than in the first step, and the growth rate of thesecond step is about one-fourth of the first step. At the end of thefirst step, the GaN surface 514 is rough and covered with the pits. Ifthe growth is stopped at the end of the first step and wafer is takenout of the reactor, the GaN surface 514 is not specular, as shown in amicrophotograph in FIG. 6. The pit coverage, defined as the percentageof a surface covered with the pits on the surface, is in one examplegreater than about 50%, and in another example greater than about 75%,and in another example greater than about 90% at the end of the firstGaN growth step. At the end of the second step, the GaN surface 518 issmooth, specular and pit-free. The pit coverage in the final film is inone example less than 1%, in another example less than 0.1%, and inanother example less than 0.01%.

The resulting GaN film 524 may have a characteristic dimension (e.g.,diameter) as large as the initial substrate 504. As examples, when a 2″substrate 504 is utilized, a 2″ GaN film 524 may be obtained. When a 3″substrate 504 is utilized, a 3″ GaN film 524 may be obtained. When a 4″substrate 504 is utilized, a 4″ GaN film 524 may be obtained. Thethicknesses of the respective GaN layers 512 and 516 grown in the twosteps is in one example in a ratio between about 2:1 and about 1:5, andin another example in a ratio between about 1:1 and about 1:3, and inanother example in a ratio between about 1:1 and about 1:2. The exactconditions of the two steps may strongly depend on the reactorconfiguration and method of temperature measurement, and may be easilyfound by those skilled in the arts. The total thickness of the GaN film524 in one example ranges from approximately 10 to approximately 250microns, in another example from approximately 10 to approximately 200microns, and in another example from approximately 20 to approximately100 microns, and in another example from approximately 20 toapproximately 50 microns.

The HVPE GaN layers 512 and 516 may be grown without intentionallyintroduced impurities. However, because of the crystal defects andresidual impurities such as oxygen and silicon from the reactor, anunintentionally doped GaN layer may still have n-type conductivity. TheGaN may also be grown with the presence of intentionally introducedimpurities such as silane or oxygen for n-type doping, or magnesium forp-type doping. When transition metal impurities are introduced, the GaNfilm 524 can be made semi-insulating. Transitional metal impurities,such as iron, may be introduced using, for example, volatilemetal-organic compounds such as ferrocene. It will be understood thatthe growth conditions may be slightly different when the dopingimpurities are introduced. In one example, the dopant concentration(e.g., n-type, p-type, transition metal, etc.) is greater than about1×10¹⁸ cm⁻². In one example of a semi-insulating GaN film 524 producedaccording to the present disclosure, the GaN film 524 has a resistivitygreater than about 1×10⁵ ohm-cm.

Because of the thermal mismatch between the substrate 504 and the GaNfilm 524, the wafer is bowed after cool-down from the growth temperatureto the ambient temperature. The bow of the wafer complicates the devicefabrication process and a large bow of the wafer is not desirable. Oneaspect of the present invention is that the GaN material during growthdevelops a tensile stress that will compensate the thermal stress andreduce the wafer bow. The tensile stress of the GaN material during thegrowth is associated with the reduction of dislocations in the GaNmaterial. In another implementation of the present invention, a thickersubstrate 504 may also be employed to reduce the GaN film bow. Inanother implementation, the backside of the substrate 504 ismechanically lapped to introduce damage on the backside of the substrate504, which reduces the bow of the GaN film 524 on the substrate 504. Inone example, the wafer bow is less than about 200 microns. In anotherexample, the wafer bow is less than about 100 microns. In anotherexample, the wafer bow is less than about 50 microns. In anotherexample, the wafer bow is less than about 25 microns. Wafer bow may bedefined as the deviation of the center point of the median surface ofthe wafer from a median-surface reference plane of the wafer.

As an example of wafer bowing, FIG. 7 illustrates a bowed wafer 704having a bowed median surface 708 with a center point 712. A mediansurface reference plane 716 with a center point 720 is established bythree equally-spaced points on the median surface at the wafercircumference. In this example, the wafer bow b, projected to the rightof the bowed wafer 704, is the distance between the center point 712 inthe median surface of a free unclamped wafer and the center point 720 inthe median surface reference plane 716. It will be understood that theradius of curvature of the bowed wafer 704 as depicted in FIG. 7 isexaggerated for illustrative purposes.

The crystal defect density, specifically, threading dislocation density,decreases with the thickness of the GaN film grown. In implementationsdescribed in the present disclosure, the lattice mismatch between theGaN material and substrate that generates dislocation is firstaccommodated by the AlN layer. The dislocations in the GaN material arefurther annihilated during the two-step GaN growth. The reduction ofdislocation density during HVPE GaN growth according to implementationsdescribed in the present disclosure is much faster than those disclosedin the prior arts. For example, U.S. Pat. Nos. 6,533,874 and 6,156,581disclose a GaN base structure grown by an HVPE process. According to theprior art, the dislocation density of a 10-micron thick GaN film grownby HVPE on sapphire is approximately 10⁹ cm⁻², and the dislocationdensity is reduced to approximately 10⁸ cm⁻² for a 23-micron GaN film,and to approximately 10⁷ cm⁻² for a 300-micron GaN film. Inimplementations of the present invention, improved GaN films have beengrown by HVPE on sapphire, as represented by the following examples: adislocation density on the surface less than 10⁸ cm⁻² for a 10-micronGaN film, less than 5×10⁷ cm⁻² for a 20-micron GaN film, and less than2×10⁷ cm⁻² for a 50-micron GaN film. The surface dislocation density ofGaN film grown according to implementations of the present invention isapproximately several factors lower than GaN films of the prior art atsimilar thickness. According to some examples of the invention, thethreading dislocation density on the surface of the GaN film may be lessthan 1×10⁸ cm⁻², in other examples less than 5×10⁷ cm⁻², in otherexamples less than 1×10⁷ cm⁻², and in other examples less than 5×10⁶cm⁻².

The wafer structure and method for making the structure of the presentinvention differ substantially from the prior art of U.S. Pat. Nos.6,533,874 and 6,156,581. We were not able to grow device-qualityepitaxial single-crystal GaN films using the methods taught by prior artsuch as in these patent references. By contrast, in accordance with thepresent invention, including the use of the epitaxial nitride templatelayer 508 (FIG. 5) described above, we can reproducibly growdevice-quality epitaxial single-crystal GaN films by HVPE. Additionally,the present invention discloses methods for eliminating GaN filmmicrocracking during HVPE GaN growth. Microcracking of GaN film duringthe HVPE growth and methods for eliminating the growth microcrackinghave not been disclosed in the prior art. Implementations of the presentinvention employ a two-step HVPE GaN growth process to eliminate thegrowth microcracking and to produce smooth surfaces on the GaN films.

Low-defect single-crystal film of Group III nitride alloys,Al_(x)Ga_(y)In_(z)N (x+y+z=1, 0<=x<=1, 0<=y<=1, 0<=z<=1), may besimilarly grown according to additional embodiments of the presentinvention. An AlN nucleation layer is first deposited on a substrate.Single-crystal Al_(x)Ga_(y)In_(z)N film is grown on the AlN-coatedsubstrate by HVPE using the two-step growth process described above. TheAl_(x)Ga_(y)In_(z)N film is grown under a condition that yields a pittedsurface morphology in the first step and then under a growth conditionthat promotes filling the pits to produce a smooth surface morphology inthe second step. Typically, the first step has a lower growthtemperature, and/or higher growth rate, and/or higher ammonia flow thanthe second growth step. The exact condition for the two-stepAl_(x)Ga_(y)In_(z)N growth depends on the reactor configuration and filmcomposition, and may be easily determined by those skilled in the art.Thus, as previously noted, the term “GaN” encompasses“Al_(x)Ga_(y)In_(z)N.”

The surface morphology of the low-defect GaN film 524 may be furtherimproved by using a chemical mechanical polish (CMP). The as-grown HVPEGaN film may exhibit some hillock features as shown in FIG. 8. In someapplications, the macroscopic roughness of the GaN film surface 518(FIG. 5) is less desirable for further device layer growth. The GaN filmsurface 518 may be improved by chemical mechanical polish. The CMPprocess does not produce surface and subsurface damage on the GaN filmsurface 518 because of the active chemical etching during the polish.

The present invention can be further understood by followingillustrative, non-limiting examples.

EXAMPLE 1 Low-Defect GaN Film Growth

In this example, we illustrate the growth of a high-quality, low-defectGaN film suitable for the further growth of electronic andoptoelectronic devices. A 2″-diameter, 430-micron thick sapphire wasused as the starting substrate. Using the sputtering method disclosed inU.S. Pat. No. 6,784,085, an AlN layer approximately 0.7 μm thick wasgrown on the sapphire substrate for use as a template layer for the HVPEGaN growth. X-ray diffraction was used to verify the AlN film wassingle-crystal. The AlN/sapphire structure was loaded into a verticalHVPE system and the GaN growth was commenced.

The HVPE GaN film was grown by a two-step method. The GaN film was firstgrown under conditions of growth rate of approximately 260 microns perhour, growth temperature of 955° C., HCl flow rate of 92 sccm, and NH₃flow rate of 2500 sccm. After growth of approximately 4 minutes underthese growth conditions, the growth rate was reduced to approximately 65microns per hour by reducing HCl flow to 23 sccm, and growth temperaturewas raised by 20° C. After growth of approximately 7 minutes under theseconditions, the NH₃ flow was further reduced to 750 sccm forapproximately 32 minutes. The total grown GaN film thickness wasapproximately 60 microns. The bow of the wafer was approximately 190microns. The GaN film was specular visually, and under opticalmicroscope observation, hillock features were present on the surface asshown in FIG. 8.

An atomic force microscope (AFM) was used to image the wafer surface andto measure the threading dislocation density. A threading dislocationterminates on the surface as a pit that can be observed with AFM. FIG. 9is a 10-micron by 10-micron AFM scan of the wafer surface. The pitdensity, i.e., the threading dislocation density on the surface, wasapproximately 1.9×10⁷ cm⁻².

EXAMPLE 2 Low-defect GaN Film Growth

In this example, we illustrate the growth of another high-quality,low-defect GaN film suitable for the further growth of electronic andoptoelectronic devices. A 2″-diameter 430-micron thick sapphire was usedas the starting substrate. Using the sputtering method disclosed in U.S.Pat. No. 6,784,085, an AlN layer approximately 0.7 μm thick was grown onthe sapphire substrate for use as a template layer for the HVPE GaNgrowth. The AlN/sapphire structure was loaded into a vertical HVPEsystem and the GaN growth was commenced.

The HVPE GaN film was grown by a two-step method. The GaN film was firstgrown under conditions of growth rate of approximately 260 microns perhour, growth temperature of 955° C., HCl flow rate of 92 sccm, and NH₃flow rate of 2500 sccm. After growth of approximately 3 minutes underthese growth conditions, the growth rate was reduced to approximately 30microns per hour by reducing the HCl flow rate to 10 sccm. At the sametime, the growth temperature was raised by 20° C. and the NH₃ flow ratewas reduced to 400 sccm for an additional 25 minutes. The total grownGaN film thickness was approximately 25 microns. The bow of the waferwas approximately 95 microns. The GaN film was specular visually, andunder optical microscope observation hillock features were present onthe surface.

EXAMPLE 3 Low-defect GaN Film Growth with Lapping Treatment

The GaN film on sapphire obtained from Example 2 is mounted on astainless steel plate using wax with the GaN film facing the plate. Thebackside of the sapphire substrate is lapped on a metal lapping platewith 30-micron diamond slurry. After removing approximately 10 micronsfrom the backside of the sapphire substrate, the wafer bow is reducedfrom approximately 95 microns to approximately 40 microns.

EXAMPLE 4 Low-Defect GaN Film Growth with Polishing Treatment

The GaN film on sapphire obtained from Example 3 is mounted on astainless steel plate using wax with the GaN film facing up. The surfaceof the GaN film is then chemical mechanically polished to removeapproximately one micron of surface material. The root-mean square (RMS)surface roughness of the GaN film is reduced from approximately 5 nm forthe as-grown film to approximately 0.5 nm or less for the CMP polishedsurface.

The examples of the present invention utilized several specific growthsequences. It should be understood that these specific growth processare meant for purposes of illustration and not to be limiting. It shouldalso be noted that growth conditions cited in the examples are specificto the HVPE growth reactor used in the examples. Different reactordesign or reactor geometry may need a different condition to achievesimilar results. However, the general trends are still similar.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the growth of low defectdensity GaN film within the scope of the present invention. Thus it isconstrued that the present invention covers the variations andmodifications of this invention provided they come within the scope ofthe appended claims and their equivalent.

1. A method for making a low-defect single-crystal gallium nitride (GaN)film, comprising: depositing an epitaxial aluminum nitride (AlN) layeron a substrate; growing a first epitaxial GaN layer on the AlN layer byHVPE under a growth condition that promotes the formation of pits,wherein after growing the first GaN layer the GaN film surfacemorphology is rough and pitted; and growing a second epitaxial GaN layeron the first GaN layer to form a GaN film on the substrate, wherein thesecond GaN layer is grown by HVPE under a growth condition that promotesfilling of the pits, and after growing the second GaN layer the GaN filmsurface morphology is essentially pit-free, wherein the GaN growthcondition for growing the first GaN layer is selected from the groupconsisting of a higher growth rate than during growth of the second GaNlayer, a lower growth temperature than during growth of the second GaNlayer, a higher ammonia flow than during growth of the second GaN layer,and two or more of the foregoing.
 2. The method of claim 1, wherein thesubstrate is sapphire.
 3. The method of claim 1, wherein the substrateis selected from the group consisting of sapphire, silicon, siliconcarbide, diamond, lithium gallate, lithium aluminate, zinc oxide,spinel, magnesium oxide, and gallium arsenide.
 4. The method of claim 1,wherein the substrate has surface orientation ranging from about 0° andabout 5° with respect to a (0001) crystal orientation.
 5. The method ofclaim 1, wherein the epitaxial AlN layer is deposited by hightemperature reactive sputtering.
 6. The method of claim 1, wherein theepitaxial AlN layer is deposited by a technique selected from a groupconsisting of sputtering, molecular beam epitaxy, metal-organic vaporphase epitaxy, hydride vapor phase epitaxy, and annealing in ammonia. 7.The method of claim 1, wherein the thickness of the deposited AlN layeris between approximately 0.05 and approximately 2 microns.
 8. The methodof claim 7, wherein the thickness of the grown first GaN layer rangesfrom approximately 2 to approximately 50 microns, and the thickness ofthe grown second GaN layer is approximately 3 microns or greater.
 9. Themethod of claim 1, wherein the GaN surface morphology after growing thefirst GaN layer is not specular and has a pit coverage greater than 50%.10. The method of claim 1, wherein the growth condition for the firstGaN layer includes a first GaN layer growth temperature ranging fromabout 900° C. to about 1000° C., a first GaN layer V:III ratio rangingfrom about 10 to about 100, and a first GaN layer growth rate rangingfrom about 50 μm/hr to about 500 μm/hr.
 11. The method of claim 10,wherein the growth condition for the second GaN layer includes a secondGaN layer growth temperature ranging from about 920° C. to about 1100°C., a second GaN layer V:III ratio ranging from about 8 to about 80, anda second GaN layer growth rate ranging from about 5 μm/hr to about 50μm/hr.
 12. The method of claim 1, wherein the thickness of the grownfirst GaN layer ranges from approximately 2 to approximately 50 microns.13. The method of claim 1, wherein the growth condition for the secondGaN layer includes a growth temperature ranging from about 920° C. toabout 1100° C., a V:III ratio ranging from about 8 to about 80, and agrowth rate ranging from about 5 μm/hr to about 500 μm/hr.
 14. Themethod of claim 1, wherein the thickness of the grown second GaN layeris about 3 microns or greater.
 15. The method of claim 1, wherein thethickness of the grown second GaN layer ranges from approximately 3 toapproximately 200 microns.
 16. The method of claim 1, wherein the totalthickness of the GaN film after growing the second GaN layer ranges fromapproximately 10 to approximately 250 microns.
 17. The method of claim1, wherein the ratio of the thickness of the grown first GaN layer tothe thickness of the grown second GaN layer ranges from approximately2:1 to approximately 1:5.
 18. The method of claim 1, wherein the GaNfilm has a characteristic dimension of about 2 inches or greater. 19.The method of claim 1, wherein the threading dislocation density on theGaN film surface after growing the second GaN layer is less than 1×10⁸cm⁻².
 20. The method of claim 1, wherein the threading dislocationdensity on the GaN film surface after growing the second GaN layer isless than 5×10⁷ cm⁻².
 21. The method of claim 1, wherein the threadingdislocation density on the GaN film surface after growing the second GaNlayer is less than 1×10⁷ cm⁻².
 22. The method of claim 1, wherein thethreading dislocation density on the GaN film surface after growing thesecond GaN layer is less than 5×10⁶ cm⁻².
 23. The method of claim 1,further including polishing the GaN film.
 24. The method of claim 23,wherein the surface root-mean square roughness of the GaN film afterpolishing is about 0.5 nm or less.
 25. The method of claim 1, furtherincluding, after growing the second GaN layer, mechanically lapping aback side of the substrate.
 26. The method of claim 1, wherein the bowof the GaN film on the substrate is less than 200 microns.
 27. Themethod of claim 1, wherein the bow of the GaN film on the substrate isless than 100 microns.
 28. The method of claim 1, wherein the bow of theGaN film on the substrate is less than 50 microns.
 29. The method ofclaim 1, wherein the bow of the GaN film on the substrate is less than25 microns.
 30. A low-defect single-crystal GaN film produced accordingto the method of claim
 1. 31. A low-defect single-crystal GaN filmhaving a characteristic dimension of about 2 inches or greater, and athickness normal to the characteristic dimension ranging fromapproximately 10 to approximately 250 microns, the GaN film including apit-free surface, the threading dislocation density on the GaN filmsurface being less than 1×10⁸ cm⁻².
 32. A low-defect single-crystalgallium nitride (GaN) on substrate structure, comprising: a substrate;an epitaxial aluminum nitride (AlN) layer on the substrate; and a GaNfilm on the substrate, the GaN film including a first epitaxial GaNgrowth layer and a second epitaxial GaN growth layer, wherein: the firstepitaxial GaN layer is grown on the AlN layer under a growth conditionthat promotes the formation of pits, and after growing the first GaNlayer the GaN film surface morphology is rough and pitted; and thesecond epitaxial GaN is grown on the first GaN layer by HVPE under agrowth condition that promotes filling of the pits formed, and aftergrowing the second GaN layer the GaN film surface morphology isessentially pit-free.
 33. The GaN on substrate structure of claim 32,wherein the substrate is sapphire.
 34. The GaN on substrate structure ofclaim 32, wherein the substrate is selected from the group consisting ofsapphire, silicon, silicon carbide, diamond, lithium gallate, lithiumaluminate, zinc oxide, spinel, magnesium oxide, and gallium arsenide.35. The GaN on substrate structure of claim 32, wherein the thickness ofthe deposited AlN layer is between approximately 0.05 and approximately2 microns, the thickness of the grown first GaN layer ranges fromapproximately 2 to approximately 50 microns, and the thickness of thegrown second GaN layer is approximately 3 microns or greater.
 36. TheGaN on substrate structure of claim 32, wherein the total thickness ofthe GaN film ranges from approximately 10 to approximately 250 microns.37. The GaN on substrate structure of claim 32, wherein the ratio of thethickness of the grown first GaN layer to the thickness of the grownsecond GaN layer ranges from approximately 2:1 to approximately 1:5. 38.The GaN on substrate structure of claim 32, wherein the GaN film has acharacteristic dimension of about 2 inches or greater.
 39. The GaN onsubstrate structure of claim 32, wherein the threading dislocationdensity on the GaN film surface is less than 1×10⁸ cm⁻².
 40. The GaN onsubstrate structure of claim 32, wherein the surface root-mean squareroughness is less than 0.5 nm.
 41. The GaN on substrate structure ofclaim 32, wherein the bow of the GaN film on the substrate is less than200 microns.